There is known a vehicle seat 201 with a side airbag system 240. In the side airbag system 240, as shown in FIGS. 7, 9, when a vehicle is collided from a side thereof, an airbag body 242 accommodated inside a seatback 203 of the collided vehicle is inflated by a gas injected from an inflator (not shown). The inflated airbag presses and opens a back pad 220 (not shown in FIGS. 7 and 9). When the airbag body 242 is inflated further with the back pad 220 being pressed and opened, the airbag member 242 deploys to an outside of the seat back 203 along a trim (not shown) provided to an inside of a side door while tearing a seat skin 203 which covers a surface of the back pad 220. Here, JP-A-2007-176349 discloses a vehicle seat 201 with a side airbag system 240 in which a deployment slit 222 is formed to a back pad 220 of a seatback 203 as shown in FIG. 8. According to the configuration disclosed in JP-A-2007-176349, an airbag body 242 can deploy reliably.